1. Field of the Invention
The present invention relates to a differential termination resistor adjusting circuit that is used for a data transmission system having a plurality of data transmission channels and, in particular, for a system requiring adjusting an input impedance in a receiving side device (Rx), and a differential termination resistor adjusting circuit that has been used in recent years for a monitor and the like having a digital I/F and that is used for building a termination resistor in a differential transmission system such as LVDS or the like, featuring high speed, low voltage, and low noise transmission, in the chip of the receiving side device (Rx).
2. Description of the Related Art
FIG. 13 is an equivalent circuit of a data transmission between different devices of a transmission system (single end transmission system) in the prior art. FIG. 14 is an equivalent circuit of a data transmission between different devices of a transmission system (differential transmission system) in the prior art. FIG. 15 is an equivalent circuit of a data transmission between different devices of a transmission system (transmission system having a matched impedance) in the prior art. In FIG. 13, FIG. 14 and FIG. 15, a reference numeral 1 denotes a transmitting side device (Tx); a reference numeral 2 denotes a receiving side device (Rx); reference numerals 3 and 4 denote transmission lines having impedance of Z0; and reference numerals 5 and 6 denote termination resistors.
Next, operation will be described.
In the data transmission, generally, when in a case that the propagation delay time in the transmission lines 3, 4 is longer than the rise up time or fall down time of a signal, it is necessary to treat the transmission line as a distributed constant line and to take into account also noises such as reflection or the like.
At this point, especially in a high speed data transmission system, in many cases, the propagation delay time in the transmission lines 3, 4 is longer than the rise up time or fall down time of a signal. Thus, it is necessary to take measures against noises such as reflection caused by the impedance mismatch between the transmission lines 3 and 4.
In contrast, to take an impedance matching of the transmission line 3 to that of the transmission line 4, in general, termination resistors 5 and 6 are used. In the case where the output impedance (Z1) of the transmitting side device 1, the characteristic impedances (Z0) of the transmission lines 3 and 4, and the values of resistances (ZL) of the termination resistors 5 and 6 are matched to each other, reflection is not fundamentally produced.
That is to say, in the high speed data transmission system, it is important to match the termination resistor to the characteristic impedance of the transmission line.
As shown in FIG. 15, however, the data bus between the transmitting and receiving devices of an ordinary high speed data transmission system constitutes, in many cases, a multi bit bus such as 8 bit bus, 16 bit bus or the like. In the multi bus like this, there is presented a problem that adding the termination resistors 5, 6 to all the input terminals of the receiving side device 2 increases the number of elements mounted on a substrate to thereby cause an increase in the cost of the system and the mounting area of the substrate.
At this point, there is presented a problem that building the termination resistors 5, 6 in the receiving side device 1, because the resisters have generally characteristics of variations by manufacturing, it increases a possibility that the resistors and thus raises causing the impedance mismatch. In addition, there is presented a problem of producing an inconvenience that building the termination resistors 5, 6 in the receiving side device 1 results in setting the resistances of the resistors at certain target values to thereby make it impossible to share the device among systems having the different characteristic impedances of transmission lines. Thus, adding the termination resistor externally to the device in accordance with the characteristic impedance of the transmission line used was more advantageous in the aspect of characteristics.
Because the transmission system in the prior art is constituted in the manner described above, there is presented a problem that building the termination resistors in the receiving side device increases variations in manufacturing the resistors and thus raises the possibility of causing an impedance mismatch. Furthermore, there is presented a problem of producing an inconvenience that building the termination resistors in the receiving side device results in setting the resistances of the resistors at certain target values to thereby make it impossible to share the device among the systems having the different characteristic impedances of the transmission lines.
The present invention has been made to solve the problems mentioned above. It is the object of the present invention to provide a differential termination resistor adjusting circuit in which an input termination resistor is built in a semiconductor device to reduce the cost of a transmission system and the mounting area of a substrate.
Further, in a differential transmission system, it is the object of the present invention to keep an impedance matching in a transmission system at the same level as compared with a case where an external resistor is used, by means of reducing variations in the resistance of the termination resistor built in the semiconductor device as compared with the prior art.
Still further, in the differential transmission system, it is the object of the present invention to make the termination resistors of all the data buses equal to the characteristic impedance of the transmission line and to vary the value of the impedance by externally mounting one reference resistor that is equal to the characteristic impedance of the transmission line.
Still further, in the differential transmission system, it is the object of the present invention to share a semiconductor device also among systems that are different from each other in the characteristic impedance of the transmission line by varying the resistance of the reference resistor.
A differential termination resistor adjusting circuit in accordance with the present invention includes: a reference current producing section that produces a nearly constant reference current irrespective of variations in power supply voltage, temperature, and manufacturing; a reference voltage producing section that produces a nearly constant reference voltage irrespective of variations in the power supply voltage, the temperature, and the manufacturing; a replica resistor producing section that is supplied with the reference current to produce a voltage drop; a control voltage producing section that produces a control voltage based on the reference voltage and the voltage drop; and a genuine resistor producing section that is built in a receiving side device and is connected to an input termination, wherein the resistances of the replica resistor producing section and the genuine resistor producing section are adjusted by the control voltage.
As described above, according to the present invention, in the differential transmission system, the resistance of the input termination resistor built in the semiconductor device can be adjusted. Therefore, it is made so as to be possible to produce an effect of reducing the cost of the differential transmission system and the mounting area of the substrate and to keep the impedance matching of the differential transmission system at the same level as compared with the case where the external resistor is used by sufficiently reducing variations in the resistance of the input termination resistor as compared with the prior art.